1. Field of the Invention
The present invention relates to a method for manufacturing a biodegradable high molecular weight aliphatic polyester that is a biodegradable and has characteristics that are applicable to conventional uses of conventional polyolefins. Further, the present invention relates more particularly to a method for manufacturing a biodegradable high molecular weight aliphatic polyester which is stable when manufacturing molded products by a variety of molding methods and is preferable for manufacturing such products as films, blow molding products, sheets, filaments and foamed products etc.
The term "biodegradable" used in this specification means a sample having the characteristics of being able to be broken down by microorganisms and losing its original form, a certain time lapse after the sample is embedded in natural soil or water.
2. Background of the Invention
From the standpoint of environmental protection in recent years, biodegradable plastics have attracted public attention, such that polymers having sufficient physical characteristics to meet practical requirements, the necessary molding properties and cost effectiveness are eagery wanted.
A type of polymer in which starch is included in polyethyrene has conventionally been realized as a film, but this could not attain the above purposes since the polyethyrene still remains as it is and even the starch portion is broke down in the soil.
A number of trials have been made to manufacture biodegradable plastics using materials produced by natural materials or organisms, some of which are practically realized ("Practical Biodegradable Plastics", CMC Publishing (1992)).
However, there have not been any biodegradable plastics with sufficient characteristics and molding properties to replace conventional general plastics.
Although it is known that aliphatic polyesters are biodegradable, at the same time they not only have the problem of instability resulted in poor molding properties, but they also have the defect of not being able to increase their molecular weights to the extent which indicates useful characteristics. Thus, no aliphatic polyesters having sufficiently high molecular weight for practical use have been known yet.
The present inventors have recently discovered a new method for a manufacturing biodegradable aliphatic polyester which is useful for a variety of applications and for manufacture of molded products. This new method comprises applying a 0.1-5 parts by weight of polyfunctional isocyanate to react with a 100 parts by weight of a polyester having a number average molecular weight of at least 5,000, preferably 10,000 and substantially hydroxyl groups as terminal groups, resulting in a number average molecular weight of at least 10,000, preferably 20,000.
However, the remaining problem that was found was that the metallic compound catalyst used in synthesizing the polyester remains and acts conversely as a decomposer for the polyester at the time of molding polyeser when maintained for a long time at a high temperature e.g. of 200.degree. C. or more, particularly when in contact with air. As a result, the polyester loses its thermal stability so as to be excessively colored and is reduced in molecular weight, sometimes resulting in a lowering of the molded product quality.
In conventional methods for manufacturing polyesters containing aromatic structures such as polyethylene terephtalate and polybutylene terephtalate. It has been known that aromatic phosphoric acid ester or phosphorous acid ester such as triphenyl phosphite is added to prevent coloring. These phosphorus compounds act to reduce the effect of the metallic compound catalyst and are added after and not during the reaction, and these are particularly in the manufacture of urethane rubber.
Namely, if any activity of the catalyst remains, transesterification would arise between the hard resin parts and the soft rubber parts, but this transesterification should be restricted to a minimum for providing rubbers of good quality rubber. Too much transesterification would render a condensation product of both components which acts to significantly degrade the characteristics of rubber. Therefore, the relationship between the phosphorus compounds and the catalyst, i.e. the used ratio of both is typically adjusted such that part oi the activity of the catalyst remains.
However, such a method as simply adding the aforementioned phosphorus compounds after the synthesizing reaction of the aliphatic polyester would not avoid the undesirable docompositional reactions such as coloring during the molding process at high temperaturs or the reductions in molecular weight.